EP 0 305 787 A1 discloses a permselective asymmetric membrane suitable for hemodialysis, hemodiafiltration and hemofiltration of blood, comprised of a hydrophobic first polymer, e.g. polyamide, a hydrophilic second polymer, e.g. polyvinylpyrrolidone, and suitable additives. The membrane has a three-layer structure, comprising a first layer in the form of dense, rather thin skin, responsible for the sieving properties, a second layer in the form of a sponge structure, having a high diffusive permeability and serving as a support for said first layer, and a third layer in the form of a finger structure, giving the membrane a mechanical stability.
WO 2004/056459 A1 discloses a permselective asymmetric membrane suitable for hemodialysis, comprising at least one hydrophobic polymer, e.g. polyethersulfone, and at least one hydrophilic polymer, e.g. polyvinylpyrrolidone. The outer surface of the hollow fiber membrane has pores in the range of 0.5 to 3 μm and the number of pores in the outer surface is in the range of 10,000 to 150,000 pores per mm2.
While these membranes already show very good performance in hemodialysis and excellent biocompatibility, there is a desire to further improve their performance to enhance removal of middle molecular weight substances, e.g. inflammatory mediators having a molecular weight between 20 and 40 kDa. It has now been found that membranes with improved performance can be obtained by a particular production process which comprises a step of treating the membranes with a salt solution before drying them.